Eliminates internal floating piston to improve suspension performance

The Trek Re:aktiv Thru Shaft is shown here in different stages of compression.

What is it

Trek is always trying to differentiate by improving the rear shock on their bikes. With their suspension ace Jose Gonzalez in an R&D lab in Southern California, they’ve been able to develop rear suspension technology that gives their bikes an edge. First came DRCV and then Re:aktiv. Now we have Re:aktiv with Thru Shaft technology.

The principle is sound, as this technology is used in the highest level of motorsports. Essentially, it eliminates the use of an internal floating piston (IFP), replacing it with a through shaft to transfer oil between two chambers.

What’s the advantage

Traditional shocks employ a secondary gas chamber IFP, compensating for the change in damper volume caused by the piston taking up room. The problem is it creates lag in the system. RE:aktiv with Thru Shaft eliminates the internal floating piston and improves responsiveness.

The thru shaft design also eliminates the gas pressure feel entirely, and gets rid of any displaced fluid, so the need for an IFP valve or second gas chamber is eliminated. This drastically reduces running friction and improves consistency by eliminating the effect of rising gas pressure commonly associated with gas shocks. Also by eliminating the IFP valve, there’s a reduction in the number of pieces and flow paths the fluid must take. This allows the shock to change direction quicker, resulting in more mechanical grip.

One of the issues all shock tuners have to deal with is the fact that as the shock compresses the rod takes up volume inside the shock body. That’s the reason there is always some type of gas inside the shock. The shock needs to have something that will compress when the rod extends into the shock body because the shock oil won’t.

Trek Re:aktiv Thru Shaft in Verbier, Switzerland.

Does it work

There’s two implementations of the Trek’s Thru Shaft technology, one by Fox on the new Trek Fuel, and one by RockShox on the new Trek Remedy. We got to try them both at the Verbier Bike Park in Switzerland.

Rear shocks now compared to 5 years ago have experienced dramatic improvement. The DPS offerings by Fox and the Monarch products by RockShox are now very supple and well damped.

In our short experience with the thru shaft, we could tell that there was indeed a difference. It wasn’t a mind blowing departure from the current standard offerings but the suspension was more supple, and it seemed to change direction more willingly.

As we entered some rocky, rooty singletrack the rear felt more planted and it was not only more comfortable than what we were used to, it also delivered more traction. Day two delivered the same experience but it was subtle like the previous day. We’d like to spend more time with these rear shocks on our home trails to truly experience the difference and eliminate variables.

Trek Re:aktiv Thru Shaft internals illustrated.

Thru Shaft Available On:

Trek Remedy 9.8

Trek Remedy 9.8 Women’s

Trek Fuel EX 9.9

Trek Slash 9.7

Trek Slash 9.8

Framesets: Carbon Fuel EX, Carbon Remedy, Carbon Slash

Trek Re:aktiv Thru Shaft on Fuel EX 29er in one of the best areas to ride in Switzerland.

RE:aktiv with Thru Shaft FAQ

Is this technology proprietary to Trek?
Trek has an exclusivity agreement with both RockShox and Fox for two years. There are no patents on Thru Shaft, so after two years, other brands and Fox or RockShox could implement parts of the design. However, Penske still owns the patent for the RE:aktiv valve, so only Trek will benefit from RE:aktiv’s regressive damping.

How does RE:aktiv with Thru Shaft compare to other all-mountain or enduro shocks like the Fox X2 or RockShox Super Deluxe?
RE:aktiv with Thru Shaft is the only damper that eliminates oil displacement, the dynamic internal floating piston (IFP), and gas charge; as a result of this elimination, RE:aktiv with Thru Shaft has reduced hysteresis (lag) and more balanced damper pressures, which allows it to more quickly and effectively react to changing terrain. Overall, it’s a much more responsive damper with the same air spring performance.

What does all of that mean? How does eliminating oil displacement benefit the rider?
In a traditional air shock, the damper shaft displaces oil as the shock moves through its stroke. The IFP, a gas-charged piston in the damper, compensates for this constant change in damper volume. As the damper rod displaces oil, the increased damper volume creates enough pressure to compress the gas charge and move the IFP. As the shock rebounds and pressure is reduced, the IFP will start floating back to its original position, and the cycle continues.

The rod pressure from the gas charge and the stick and slip effect of the IFP’s movement create hysteresis, or lag, which keeps the shock from working as quickly as possible.

By eliminating oil displacement, we also eliminate the need for a dynamic IFP. With no dynamic IFP, hysteresis is dramatically reduced, which creates a damper that reacts to changing terrain significantly faster than anything else available.

Trek Re:aktiv Thru Shaft line at Swiss Alps

How does RE:aktiv with Thru Shaft eliminate oil displacement?
Rather than a single damper shaft that displaces oil as it moves deeper into the stroke, Thru Shaft uses a shaft on either side of the damper valve that moves through a single, solid column of oil. As the main shaft enters the damper, the secondary shaft exits the damper on the other side. Conversely, as the main shaft exits the damper, the secondary shaft enters the damper on the other side. This results in a constant damper volume with no displacement and more balanced internal pressure.

How does all this affect the rider?
It’s a more responsive shock. With no IFP force acting against the damper shaft, small-bump sensitivity is greatly improved. Eliminating the dynamic IFP also eliminates its friction and stick and slip effect, so not only is the shock movement easier to initiate, it also changes direction much faster. The solid column of oil and immediate pressure balance result in more support and efficiency with faster response to terrain throughout the stroke. In total, this shock amplifies the responsiveness of a standard RE:aktiv shock, and keeps your rear tire glued to the trail so you can ride with even more confidence.

With no dynamic IFP, how does the shock manage heat-induced fluid expansion?
The longer-stroke RockShox version uses an external reservoir for thermal compensation. Since the shorter-stroke Fox version has less total oil volume, Fox was able to include a thermal compensator within the main damper shaft. Due to the use of a flow control check valve, thermal compensation on RE:aktiv with Thru Shaft takes place during moments when the shock is static. This eliminates the need for a dynamic IFP function and an associated IFP gas charge, which is necessary for traditional dynamic IFP shock function.

Why is RE:aktiv with Thru Shaft only available on the highest-end bike models? Will it make its way onto other models?
Initially, we are only offering RE:aktiv with Thru Shaft on the highest-end models because of development and production costs. We expect to offer it on more models in the future.

Can riders upgrade other Trek mountain bikes with this new shock?
No, we are not offering the new RE:aktiv with Thru Shaft shocks as an aftermarket upgrade at this time.

Is Thru Shaft something that we could see on DH or XC bikes?
We’re currently focused on using RE:aktiv with Thru Shaft in trail bike applications, as that is where we see the greatest performance benefits. However, as with any other new technology, we’re always exploring other potential applications.

Is there a weight limit on RE:aktiv with Thru Shaft?
Since weight limitations are based on the air spring, and not the damper, the weight limits are the same as with the standard offerings from either manufacturer.

Is the setup process the same as traditional air shocks?
Yes. Riders will still set air pressure based on desired sag, and adjust the rebound damping accordingly.

Trek Re:aktiv Thru Shaft on Slash 29er in Verbier, Switzerland.

Can riders adjust the air spring volume?
Yes. As with other modern air shocks, riders can add volume spacers to both Fox and RockShox Thru Shaft shocks for a more progressive spring rate.

Is the new Thru Shaft shock user serviceable?
Yes. Recommended maintenance for Thru Shaft shocks is similar to traditional IFP shocks.

The founder of mtbr and roadbikereview, Francis Cebedo believes that every cyclist has a lot to teach and a lot to learn. "Our websites are communal hubs for sharing cycling experiences, trading adventure stories, and passing along product information and opinions." Francis' favorite bike is the last bike he rode, whether it's a dirt jumper, singlespeed, trail bike, lugged commuter or ultralight carbon road steed. Indeed, Francis loves cycling in all its forms and is happiest when infecting others with that same passion. Francis also believes that IPA will save America.

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